In Long Term Evolution (LTE), a scheme is specified in which a mobile station (UE) carries out LTE voice calls (VoLTE) and the like by establishing an IP-CAN Bearer (SIP bearer), which is a logical communication path for Session Initiation Protocol (SIP) and voice media, on an IP-Connectivity Access Network (IP-CAN). IP-CAN is an IP access network providing a function to transport SIP and voice media (see, for example, Non-patent document 1)
It is also specified that the mobile station executes procedures for connection to the IP-CAN when the IP-CAN Bearer is released for some reason (for example, handover of the mobile station to UTRAN (3G)/GERAN (2G), an overload state on the network side, or the like) (see, for example, Non-patent document 2).
FIG. 1 shows a communication sequence in a case where, after the mobile station establishes an IP-CAN Bearer via E-UTRAN (LTE), the mobile station performs handover (transition) to another radio access network UTRAN/GERAN and thereby the IP-CAN Bearer is released. As shown in FIG. 1, when the mobile station executes handover to UTRAN/GERAN (S10), the IP-CAN Bearer is released (removal) (S20). Then, after the execution of the handover, the mobile station executes procedures for connection to the IP-CAN (S30).